EUV lithography is presently being pursued as the next generation lithography process for fabricating semiconductor circuits with linewidths below 50 nm. The EUV electromagnetic radiation has a wavelength typically in the range from 10 nm to 15 nm (e.g., 13.5 nm). While EUV lithography enables printing extremely fine features without diffraction-related distortions, it requires a set of high-precision-mirrors that combine high shape accuracy with extremely low surface roughness at the atomic level.
In addition to these stringent optical requirements, the high-precision mirrors must be compatible with operation in a high-vacuum environment. Where the high-precision mirrors are used as EUV collectors in an EUV collector system, they need to withstand an aggressive plasma environment where high thermal fluxes are present. As a consequence, the EUV mirrors must be fabricated using (ultra) high vacuum compatible materials and techniques, while also having thermal management capability for certain applications.
One common method of forming a high-precision EUV mirror is to machine and polish a relatively thick substrate. For example, a typical polished silicon, ZERODUR or silicon carbide mirror has a substrate thickness of about 5 cm to 10 cm. Another method of forming a high-precision mirror is to form a thin shell-type mirror, such as by electroforming. An electroformed mirror shell has a thickness of about 1 mm to 2 mm. These two mirror types have significantly different thermal properties, such as thermal mass, thermal conductance and coefficient of thermal expansion, as well as different mechanical properties, with the most notable being structural rigidity.
An advantage of forming a high-precision mirror using a relatively thick substrate is that its thermal and mechanical properties are generally more desirable than those of an electroformed mirror shell, which typically requires a web-like mechanical support structure. On the other hand, an advantage of forming a high-precision electroformed mirror is that the surface shape can easily be replicated using a mandrel, and the surface can be made to the high precision needed to meet stringent optical performance requirements.
Accordingly, there is a need for EUV mirror systems and methods whereby a relatively thin electroformed mirror can be used in an optical system, such as a EUV lithography system, designed to employ relatively thick conventional mirrors.